Goal:
  
  A : Type
  sizeA : A → nat
  B : Type
  sizeB : B → nat
  l : list A
  f : nat → A → B
  H : ∀ (i : nat) (x : A), sizeB (f i x) = sizeA x
  ============================
   (∀ (i : nat) (x : A), sizeB (f i x) = sizeA x)


DEBUG:Going to execute:
easy
DEBUG LTAC Evaluated term: list_size_mapi_rec_eq